This invention relates to a fluidized bed heat exchanger and, more particularly, to such a heat exchanger having a fluid-cooled, air distribution plate assembly for supporting fluidizable particulate material which is insulated from the surrounding heat.
The use of fluidized beds has long been recognized as an attractive way of generating heat. In a typical fluidized bed arrangement, air is passed through a perforated plate, or the like, which supports a bed of particulate material, usually including a mixture of inert material, a fuel material such as high sulfur bituminous coal, and an adsorbent material for the sulfur released as a result of the combustion of the fuel material. As a result of the air passing through the bed, the bed behaves like a boiling liquid which promotes the combustion of the fuel. The basic advantages of such an arrangement include a relatively high heat transfer rate, substantially uniform bed temperature, combustion at relatively low temperatures, ease of handling the fuel materials, a reduction in corrosion and boiler fouling and a reduction in boiler size.
In the fluidized bed combustion process, in order to insure capture by the adsorbent of the sulfur released during combustion of the fuel, the bed temperature has to be maintained at a relatively high value (normally 1500.degree.-1550.degree. F.). This requires the bed-supporting plate to be constructed of an expensive material, such as stainless steel, which will withstand these temperatures. However, even when these type of materials are used, the large differential expansion involved and the thermal gradients occurring across the surface of the plate reduce the life of the plate considerably. As a result, the plate has to be frequently replaced, which is costly both from a material and a labor standpoint, and which results in a considerable amount of down-time. Also, in fluidized bed heat exchangers, using water-cooled walls, the difference in temperatures between the walls and the bed-supporting plate result in border seal junction problems.
According to U.S. patent application Ser. No. 138,807, filed Apr. 9, 1980, by Robert D. Stewart, and assigned to the present assignee, now abandoned, a plurality of air distributors extend perpendicular to the perforated plate assembly and register with the openings in the latter assembly for receiving and discharging air in an upward direction to fluidize the particulate material. The lengths of the air distributors are such that the upper surface of the plate assembly is insulated from the high temperatures occurring in the bed. Also, the plate assembly is formed by a plurality of tubes through which water is passed to provide further cooling. This enables the components forming the plate assembly to be fabricated at a relatively low cost, low alloy material and yet enjoy a relatively long life when compared to plates of more expensive heat-resistant material that are not so insulated and cooled.
However, it was found that the lower surface of the plate assembly and, more particularly, the lower surfaces of the tubes and fins forming the plate assembly were also subjected to a relatively high, and potentially damaging temperature by virtue of being exposed to the preheated fluidizing air passing from the air plenum through the plate assembly.